1. Field of the Invention
The present invention relates generally to a travelling wire electrical discharge machining apparatus, and more particularly to an improved electrical current pick-up therefor.
2. Description of the Related Art
An electrical discharge machining apparatus removes material from a metallic workpiece by spark erosion as a travelling wire electrode is fed from a spool or similar supply through the workpiece. The wire is also advanced relative to the workpiece in a direction generally transverse to the wire. During the machining operation, it is necessary to deliver electrical current to the travelling wire electrode. An electrically conductive current pick-up in physical contact with the travelling wire is employed to transfer current from a power supply to the wire at a point proximate the workpiece. One form for such a current pick-up is a cylinder having a central bore through which the wire electrode passes.
To maintain constant electrical contact between the current pick-up and the travelling wire electrode, wire guides for guiding the wire electrode through the bore of the current pick-up are used. The center line of the guides is offset laterally relative to the center line of the bore of the current pick-up, causing the wire electrode to bear against the inner surface of the bore.
During machining, the wire electrode is travelling at approximately 20 cm/minute across the inner peripheral surface of the central bore of the current pick-up. The inner peripheral surface of the current pick-up is therefore subjected to wear by a combination of mechanical and electrical forces. To extend the useful life of the current pick-up, it is typically constructed of an extremely hard and wear-resistant material such as carbide. Eventually, however, the travelling wire electrode erodes a slot in the inner peripheral surface of the carbide current pick-up at the point of contact to a sufficient extent that electrical conduction between the carbide current pick-up and the wire electrode is detrimentally affected. Upon that occurrence, the current pick-up is ordinarily at the end of its useful life span, and must ordinarily be replaced, even though most of the inner peripheral surface of the bore of the current pick-up is still in good condition.
It would be desirable to provide an improved current pick-up arrangement wherein the useful life span of the current pick-up could be extended by making use of the unworn portions of the current pick-up after a portion has become worn beyond usefulness. This and other desires are satisfied by the present invention.